Evaporative demand is basically the pressure the atmosphere puts on plants and soil to draw water through evaporation. Evaporative demand can tell us much about drying power we’re seeing over almost any time period we’re interested in tracking. Often, we look at the Water Year, which runs from October 1 to September 30th. Usually the fall and winter are periods when you have very little plant growth and consequently very little water usage, which helps recharge the soil for the growing season starting in the spring.
This year, we have seen very little evaporative demand. Some places, for example, like the Central to Northern Plains, that have experienced extreme flooding are seeing evaporative demand conditions that are way outside the norm. In some cases, they are seeing values that occur only about 5% to 2% of the time going back to 1978.
What this means is that soil moisture has reached its capacity and we will continue to see high runoff efficiencies into streams and rivers, which could make flooding worse. In other places that have not experienced flooding, good soil moisture will likely mean good grass production and a good grazing year for livestock.
The below interactive chart shows the Evaporative Demand Drought Index, that was developed by researchers at NOAA. You can move your mouse over each county to see what the 2019 Water Year looks like for your area.
Evaporative demand for the period between October 1st. to March 27th for 2018 and 2019. The color scale and the frequency these conditions have been observed since 1978 is below.